1. Field of the Invention
The present invention relates generally to a field of a disk drive of a perpendicular magnetic recording system, particularly to a servo writing method in which servo data is recorded on a disk medium by perpendicular magnetic recording.
2. Description of the Related Art
In recent years, practical use of a disk drive of a perpendicular magnetic recording system has been pushed forward. In the perpendicular magnetic recording system, a disk drive using a write head called a single pole type head (SPT) suitable for perpendicular magnetic recording, and a disk medium (hereinafter referred to simply as a disk) which is a double-layered perpendicular recording medium is promising.
In general, servo data for use in a control for positioning heads is recorded on disks before shipping the disk drives. In a servo writing step for recording the servo data, a servo write-only device called a servo track writer (STW) is used.
The servo data is roughly classified into an address code and a servo burst signal. The address code includes a cylinder code and a sector code.
Since the servo data is used for correctly positioning the head at a target position on the disk, a high-precision writing method is required. In the disk drive, the servo data recorded on the disk is read by a read head at the time of positioning the head. At this time, precision in positioning the head depends on precision in reproducing the servo data.
For example, when the servo burst signal included in the servo data is recorded, and when erase bands exist on opposite sides, the signal-to-noise (S/N) ratio in an operation for reproducing the servo data sometimes drops because of losses by the erase bands. To solve this problem, a technique has been proposed in which the track width of the servo burst signal is expanded to improve the S/N ratio (see U.S. Pat. No. 6,023,389, for example).
In the disk drive of the perpendicular magnetic recording system, the servo data is recorded on the disk by the write head using an SPT suitable for the perpendicular magnetic recording.
Additionally, in the perpendicular magnetic recording system, a phenomenon has been confirmed in which a recorded magnetized state changes, when a recording magnetic field having a polarity opposite to a magnetization polarity magnetically recorded on the disk beforehand is applied by the write head using the SPT. This phenomenon is called a hard-easy transition shift (hereinafter sometimes abbreviated as HETS) or simply a hard transition.
In HETS, the recording magnetic field from the head apparently becomes strong, and the magnetization displacement point shifts, when the magnetization having a pole opposite to that of the magnetization before the recording on the disk is written.
It has been confirmed that when an influence of HETS phenomenon is exerted at the time of a servo writing operation and, for example, when the servo data is written in a radial direction from an inner periphery to an outer periphery on the disk, the servo data shifts in a reverse direction, and is recorded. When the servo data is recorded in this manner, a reproduction error of the cylinder code is generated, or the precision in reproducing the servo burst signal deteriorates. As a result, the precision in positioning the head deteriorates.